1. Field of the Invention
The invention relates to a spreading device to spread fiber filament bundles to form a flat fiber band. The spreading device according to the present invention is particularly suited for use in a method for manufacturing a preform for a load path aligned fiber composite structure. Moreover, the invention relates to a spreading method carried out using such a spreading device.
2. Background Information
At the construction of vehicles of all kinds, particularly at the construction of aircrafts and spacecrafts, but also in other branches of industry such as mechanical engineering, there is an increasing need for strong and yet lightweight, cost-efficient materials. Especially fiber composite materials offer an outstanding lightweight construction potential. The principle resides in the fact that particularly high-strength and stiff fibers are embedded in a matrix in a load path aligned fashion, thus producing components having outstanding mechanical properties by using previous techniques and having a weight which at a comparable performance is typically 25% less than that of aluminum structures and 50% less than steel structures. A drawback is the high material costs and particularly the laborious and mainly manual fabrication.
Accordingly, there is a desire for an automated manufacture facilitating machine positioning of the fibers in space. Nowadays, fiber-reinforced plastic materials are characterized by an extremely high strength and stiffness at a low weight, particularly if oriented long fibers, for instance carbon fibers, are used. They also have a high weight-specific energy absorption potential and good fatigue characteristics.
Up to now this is achieved by endless fibers being incorporated in a matrix (e.g. epoxy resin) in a load path aligned fashion. Depending on the direction of reinforcement, anisotropic materials having direction-dependent mechanical properties can be produced. For instance, a material can have characteristics which are different from each other in the length and in the width of the material. Already today, a high percentage of the structural weight In modern aircrafts and spacecrafts, is made up of fiber-reinforced plastic materials.
Currently, the most important manufacturing process is based upon the so-called prepreg technology. This technology involves positioning the reinforcing fibers in a parallel (unidirectional) fashion and embedding the fibers in a matrix. After a curing step, semi-finished products are produced which are rolled up as a thin layer. During processing, these layers are cut corresponding to the contour of the component and are laminated in a tool layer by layer and preferably by hand. Thereafter, curing takes place under pressure and temperature inside an autoclave. The resulting components exhibit a very high light construction potential, but the manufacture is laborious and expensive. For this reason material searchers have for long dealt with the question in which way fibers can be positioned aligned to the load path and three-dimensionally and with a contour which matches the final contour of the component as closely as possible, in an automated process.
To produce fiber composite structures with load path aligned fibers, so-called preforms as textile semi-products have been manufactured up to present for selected applications in addition to prepregs. These are mostly two- or three-dimensional structures having a load path aligned fiber orientation. Up to present endless fibers are placed in the load direction and prefixed by using means and techniques from textile engineering, normally sewing, knitting or the like. Examples of devices and processes for producing such preforms are disclosed in DE 30 03 666 A1, DE 196 24 912, DE 197 26 831 A1 and DE 100 05 202 A1.
However, the known processes for manufacturing preforms are complicated concerning their implementation and process technique. Particularly for components where curved load path lines with a varying density are to be expected, it is not possible with previous processes to manufacture a correspondingly load path aligned component. Particularly, the fibers cannot be oriented arbitrarily along defined curved paths and the fiber content cannot be locally varied.
For manufacturing the textile semi-finished parts, so called rovings are interwoven to form the textile preform by using the above explained preform manufacturing techniques. For example 12 k rovings with 12000 single filaments are used. A uniform penetration of such rovings by the material of the matrix is very complicated to accomplish. Also, at the location of the rovings high fiber concentrations exist with only a low fiber moiety in between, so that it is difficult to vary the rate of fibers locally according to the individual requirements of the component.
Different spreading techniques for spreading fiber filament bundles are known in textile engineering for completely different fields of application. In FIG. 4, the basic principle of a conventional spreading technique known from DE 715 801 A is shown. Here, a fiber strand 14 consecutively passes a bent rod 76 and then a straight rod 78. The combination of a straight and a bent rod in this known radius spreaders as shown in FIG. 4, causes a redirection of the tension force acting on the fiber. Now also a force is effective that presses the fiber onto the bent rod. At the highest point of the deflection the highest force acts on the filaments. The force decreases with an increasing distance from this point, i.e. the filaments can evade this load if moving outwardly on the bent rod. However, the result of the spreading operation is dependent on the tension force acting on the fiber, the friction between the fiber and the rod, the position of the rods relative to each other and the bending of the rod. If the bending is extreme, the difference of the acting forces between the highest point and an outer position is high to an extent that the surface friction of the rod is no longer important. The filaments will abruptly move outwardly, i.e. the fiber strand 14 would slip off or split. If the bending is too low, the bending ratio will be too low. Thus the result of the spreading operation is very irregular with an irregular fiber distribution. In particular, the result of the spreading operation is very much dependent on the quality of the material.